1. Field of the Invention
The present invention relates to optical communication devices, and more particularly, to an optical communication device for multiplexing optical signals for communication.
2. Description of the Related Art
In WDM (Wavelength Division Multiplexing) transmission, optical signals are controlled to a constant level by variable optical attenuators (hereinafter VOAs) before being output to a WDM transmission line. Thus, the optical attenuation characteristics of the VOAs are of importance.
FIG. 14 is a block diagram of a conventional optical communication device. As illustrated, the optical communication device comprises wavelength converter units 201a, 201b, . . . , 201n, VOAs 202a, 202b, . . . , 202n, an optical multiplexer (OPT-MUX) 203, an optical amplifier (WDM-AMP) 204, a filter 205, a photodiode (PD) 206, an A/D converter 207, a level comparator 208, an amplifier (AMP) setting unit 209, and a VOA controller 220. The VOA controller 220 includes a VOA control data generator 221, a D/A converter 222, and a VOA driver circuit 223. The figure also shows the optical levels of optical signals output from individual elements, wherein λ1, λ2, . . . , λn indicate the wavelengths of the respective optical signals.
The wavelength converter units 201a, 201b, . . . , 201n convert input signals, which are to be transmitted to a target of communication, to optical signals of respective wavelengths and output the resulting signals.
Under the control of the VOA controller 220, the VOAs 202a, 202b, . . . , 202n attenuate the optical signals output from the respective wavelength converter units 201a, 201b, . . . , 201n and output the resulting signals.
The optical multiplexer 203 multiplexes the optical signals output from the VOAs 202a, 202b, . . . , 202n and outputs the multiplexed signal.
The optical amplifier 204 amplifies the multiplexed optical signal output from the optical multiplexer 203, and outputs the amplified signal to a WDM transmission line, whereby the optical signal is transmitted to the target optical communication device.
The filter 205 separates the optical signal input to the optical amplifier 204 into signals of the respective wavelengths.
The photodiode 206 detects (as electrical signals) the optical levels of the optical signals of the respective wavelengths separated by the filter 205.
The A/D converter 207 subjects the optical levels detected by the photodiode 206 to analog-to-digital conversion.
The level comparator 208 compares a set value set by the AMP setting unit 209 with the individual signal values output from the A/D converter 207, and outputs the differences obtained to the VOA control data generator 221.
The AMP setting unit 209 is input with information including the number of wavelengths handled by the optical communication device, device configuration, etc. Based on the input information, the AMP setting unit 209 sets an optical level for the optical signals of the respective wavelengths to be input to the optical amplifier 204. The set optical level is output to the optical amplifier 204 and the level comparator 208.
The VOA control data generator 221 generates, based on the differences output from the level comparator 208, VOA control data for controlling the respective VOAs 202a, 202b, . . . , 202n. 
The D/A converter 222 subjects the data output from the VOA control data generator 221 to digital-to-analog conversion.
Based on the analog signal output from the D/A converter 222, the VOA driver circuit 223 controls the VOAs 202a, 202b, . . . , 202n. 
Namely, in the illustrated optical communication device, the level comparator 208 compares actual optical levels of the respective wavelengths input to the optical amplifier 204 with the optical level set by the AMP setting unit 209, and outputs the differences obtained to the VOA controller 220. The VOA controller 220 controls the VOAs 202a, 202b, . . . , 202n so that the respective differences may become “0” (zero). This permits the optical amplifier 204 to be input with optical signals with an optical level set by the AMP setting unit 209.
Meanwhile, a multichannel optical variable attenuator has been proposed whereby, even in the case where multi-wavelength signal beams develop an output tilt due to the transmission characteristics of the optical fiber transmission line or due to the gain tilt of the optical amplifier, the output tilt can be collectively compensated for with the use of a simple construction and control so that the individual powers of the multi-wavelength signal beams may be kept nearly constant (e.g., Unexamined Japanese Patent Publication No. 2000-180803).
Even in this attenuator, the optical levels of respective wavelengths are subject to variations attributable to the VOA characteristics, causing differences in dynamic range between the optical levels of the respective wavelengths. Consequently, the distance over which a certain wavelength with the smallest dynamic range can be transmitted determines the transmission distance of the WDM system, giving rise to the problem that the transmission distance of optical signals shortens.
FIG. 15 illustrates variations in the optical level among wavelengths. In the figure, parts A to D, which correspond to the respective sections indicated by the same letters in FIG. 14, respectively indicate the output optical levels of the wavelength converter units 201a, 201b, . . . , 201n, the output optical levels of the VOAs 202a, 202b, . . . , 202n, the output optical level of the optical multiplexer 203, and the output optical level of the optical amplifier 204, all appearing in FIG. 14.
As shown in the part A of FIG. 15, the optical signals output from the wavelength converter units 201a, 201b, . . . , 201n have optical levels varying according to wavelengths. The optical levels are adjusted by the VOAs 202a, 202b, . . . , 202n as shown in the part B, and then the optical signals of the respective wavelengths are multiplexed by the optical multiplexer 203, inclusive of variations attributable to the VOAs 202a, 202b, . . . , 202n, as shown in the part C. Subsequently, as shown in the part D, the multiplexed signal is amplified, together with the input variations, by the optical amplifier 204.
Variations in the optical level among the wavelengths can be suppressed to a certain degree through the control of the VOAs 202a, 202b, . . . , 202n, but the optical level variations still remain because of the individual characteristics of the VOAs. If the input to the optical amplifier 204 involves wavelength-dependent variations of optical levels, the optical level variations also are amplified and output. Consequently, the transmission distance is determined by the lowest of the input levels input to the optical amplifier 204, with the result that the transmission distance shortens.
For example, let us suppose the variations of the optical levels input to the optical amplifier 204 are so large that the average output optical level among the wavelengths is 2.0 dBm/ch while the lowest level is 0 dBm. In this case, the transmission distance is dependent on the lowest level of 0 dBm. A 2.0-dB loss of the output optical level of the optical amplifier 204 results in shortening of the transmission distance by 10 km on the assumption that the optical fiber has a transmission loss of 0.2 dB/km.
FIG. 16 shows an exemplary VOA characteristic, Wherein it is assumed that the optical attenuation by means of the VOAs 202a, 202b, . . . , 202n can be controlled, for example, in 1 mA steps. As seen from the figure, where the VOA driving current is varied from 9 mA to 10 mA, the amount of optical attenuation changes by 2.0 dB.
FIG. 17 shows, in enlargement, the part of the VOA characteristic close to a minimum loss shown in FIG. 16. As seen from the figure, where the VOA driving current is varied from 26 mA to 27 mA, the optical attenuation amount changes by 0.5 dB.
FIG. 18 shows changes in the optical attenuation amount relative to changes in the VOA driving current. Even for a 1-mA change of the VOA driving current, the consequent change of the optical attenuation amount differs depending on the magnitude of the VOA driving current, as explained above with reference to FIGS. 16 and 17. Thus, in a region where the optical attenuation amount sharply varies, the optical attenuation amount cannot be set to the expected value.
Namely, the VOA characteristic includes a region where the optical attenuation amount sharply varies (the gradient is large) and a region where the optical attenuation amount gently varies (the gradient is small), as shown in FIGS. 16 to 18, which leads to variations in the optical level among wavelengths.